Paramyxoviruses are a family of negative-sense single stranded RNA viruses that account for many animal and human deaths worldwide each year. The paramyxoviruses include sub-families Paramyxovirinae and Pneumovirinae. Respiratory syncytial virus (RSV) is an enveloped non-segmented negative-strand RNA virus in the family Paramyxoviridae, genus Pneumovirinae. It is the most common cause of bronchiolitis and pneumonia among children in their first year of life. RSV also causes repeated infections including severe lower respiratory tract disease, which may occur at any age, especially among the elderly or those with compromised cardiac, pulmonary, or immune systems. Passive immunization currently is used to prevent severe illness caused by RSV infection, especially in infants with prematurity, bronchopulmonary dysplasia, or congenital heart disease. Despite the burden of RSV infection in certain populations, development of an effective RSV vaccine remains elusive.
Parainfluenza virus (PIV) is another enveloped non-segmented negative-strand RNA virus that, like RSV, is in the paramyxovirus family. However, PIVs are in subfamily Paramyxovirinae. PIVs include members of the genus respirovirus (including PIV1, PIV3, Sendai virus) and rubulavirus (including PIV2, PIV4, PIV5). In addition the members of genus avulavirus (including Newcastle disease virus NDV) historically were termed PIVs and operationally can be considered the same. The human parainfluenza viruses (HPIVs, serotypes 1, 2, and 3) are second only to RSV in causing severe respiratory infections in infants and children worldwide, with HPIV3 being the most important of the HPIVs in terms of disease impact. The HPIV genome is approximately 15.5 kb, including a gene order of 3′-N-P-M-F-HN-L. Each gene encoding a separate mRNA that encodes a major protein: N, nucleoprotein; P, phosphoprotein; M, matrix protein; F, fusion glycoprotein; HN, hemagglutinin-neuramindase glycoprotein; L, large polymerase protein. The P gene contains one or more additional open reading frames (ORFs) encoding accessory proteins. Similar to RSV, development of an effective HPIV vaccine remains elusive.